Azeotropic Research Articles

Economic, entropy generation and environmental analysis of separation of high-concentration azeotropic mixtures by an innovative extractive distillation configuration based on multi-objective optimization

The process design and optimization of separating azeotropic mixtures by extractive distillation (ED) has been a hot problem in recent years. Conventional ED process often faces challenges of high energy consumption and high costs when dealing with azeotropic mixtures containing a high content of a certain component. Therefore, an energy-efficient innovative three-column extractive distillation process (ITCED) that is capable of simultaneously performing preconcentration and extractive distillation functions within a complex distillation column for separating ethyl acetate/n-propanol/water azeotropic mixture containing a large amount of n-propanol is developed. Multi-objective optimization is conducted with the minimization of total annual cost (TAC), CO2 emissions, and entropy generation as the objective functions. According to the optimization results, the ITCED process exhibits substantial advantages compared to the conventional three-column extractive distillation process (TCED), with reductions of 34.95% in TAC, 35.84% in CO2 emissions, and 38.91% in entropy generation. Subsequently, heat pump and heat integration technologies are employed to enhance the ITCED process for the further energy conservation. As a result, compared with the TCED process, the DHP-HI-ITCED process achieves savings of 48.18% in TAC, 67.5% in CO2 emissions and 72.96% in entropy generation.

A novel reactive-extractive distillation process for separation of water/methanol/tetrahydrofuran mixtures

The design of separation systems for the purification of azeotropic mixtures is of great importance in the chemical industries from economic and environmental points of view. Two novel reactive-extractive distillation (RED) processes, new design (I) and new design (II), were proposed for separating the azeotropic mixture of water/methanol/tetrahydrofuran (THF). These processes were compared to a conventional extractive distillation (ED) process. New design (I) employs dimethyl sulfoxide as a solvent, while new design (II) utilizes ethylene glycol. Ethylene oxide was introduced to the first column in both designs, enabling the in-situ production of ethylene glycol, a valuable byproduct. This is a novel solution to separate water from the azeotropic mixture by reaction between ethylene oxide and water. Aspen Plus software was used to simulate and design the processes. Both suggested designs were compared economically with the base design which is an ED process. According to the results, the new design (I) is more cost-effective and environmentally friendly alternative to the base design and the new design (II). It has a lower total capital cost and produces less carbon dioxide. Additionally, it generates a valuable by-product, ethylene glycol, which can be sold for substantial revenue. As a result, the new design (I) is the preferred choice for replacing the conventional extractive distillation method.

Energy-saving design and optimization of pressure-swing-assisted ternary heterogenous azeotropic distillations

A huge amount of energy is always consumed to separate the ternary azeotropic mixtures by distillations. The heterogeneous azeotropic distillation and the pressure-swing distillation are two kinds of effective technologies to separate heterogeneous azeotropes without entrainer addition. To give better play to the synergistic energy-saving effect of these two processes, a novel pressure-swing assisted ternary heterogeneous azeotropic distillation process is proposed firstly. In this process, the ternary heterogeneous azeotrope is decanted into two liquid phases before refluxed into the azeotropic distillation column to avoid the aqueous phase remixing, and three columns’ pressures are modified to decrease the flowrates of the recycle streams. Then the dividing wall column and heat integration technologies are introduced to further reduce its energy consumption, and the pressure-swing assisted ternary heterogeneous azeotropic dividing wall column and its heat integration structure are achieved. Genetic Algorithm procedure is employed to optimize the proposed processes. The design results show that the proposed processes have higher energy efficiencies and lower CO2 emissions, compared with the published ternary heterogeneous azeotropic distillation process.

Separation of n-propanol - n-propyl acetate azeotropic system by extractive distillation with ionic liquids: COSMO-RS prediction, experimental investigation and quantum chemical calculation

During the preparation of n-propyl acetate (NPAC), an azeotropic mixture of NPAC and n-propanol (NPA) will form inevitably, which is difficult to separate by ordinary distillation. In this work, COSMO-RS model was applied to screen appropriate ionic liquids (ILs) to separate the binary azeotrope of NPAC - NPA. 1-decyl-3-methylimidazolium acetate ([DMIM][Ac]), 1-octyl-3-methylimidazolium acetate ([OMIM][Ac]), and 1-hexyl-3-methylimidazolium acetate ([HMIM][Ac]) were selected as entrainers based on solvent capacity and selectivity. The vapor–liquid equilibrium (VLE) data of the binary system (NPA - NPAC) and the ternary system (NPA - NPAC - IL) were measured at atmospheric pressure. The thermodynamic consistency of VLE data was verified by van Ness test and Wisniak's L-W test. The NRTL (non-random two liquids) model was used to correlate the VLE experimental data and obtained good applicability. The experimental results demonstrated that all three ionic liquids can improve the relative volatility of NPAC to NPA, and the separation effect is more obvious with the increase of ionic liquid content. The separation performance order of three ionic liquids follows: [HMIM][Ac] > [OMIM][Ac] > [DMIM][Ac]. Finally, ESP analysis, mutual penetration analysis, and IGMH analysis plus QTAIM topological analysis were used to explore the mechanism of separation. The analysis results indicate that the interaction between ionic liquids and NPA is stronger than that between ionic liquids and NPAC. In addition, the stronger interaction between [DMIM][Ac] and NPA is due to their strong hydrogen bond (HB) interactions.

3D structure enhancers based on functionalized MIL-53(Al) for improved dimethyl carbonate/methanol pervaporative separation

Dimethyl carbonate (DMC) is considered an alternative, green solvent. This paper focused on enhancement in membrane performance in DMC removal by pervaporation (PV) from azeotropic mixture of DMC/methanol as a consequence of the incorporation of nanofiller into a PDMS matrix. Engineering a hydrophobized analogue of MIL-53(Al) (NHOCOCF3-MIL-53(Al)) as nanoenhancers for improved membrane materials is presented. XRD analysis confirmed the successful synthesis of MOFs and proved that modification does not influence crystalline structure of MOF, which is well retained. Properties of the membranes in PV were assessed employing separation factor (β) and thickness-normalized Pervaporation Separation Index (PSIN). Modified PDMS membranes possess better separation properties compared with pristine one. Results revealed that the incorporation of 5 wt% of NHOCOCF3-MIL-53(Al) caused an increase of β from 3.1 to 3.7, a significant value for organic-organic PV. Essential part of the work was to analyze impact of the presence of water in the feed on overall membrane effectiveness. It was observed that in the case of traces amount of water in the feed solution, water was preferentially transported from the feed to the permeate side. However, the transport of water through membranes was partially suppressed when water content in the feed was over 0.7 wt%.

Enhanced azeotropic C2ClF5/C3F8 separation by TMA+ exchanged MFI zeolites for ultrapure fluorine electronic gas

AbstractThe demand for the azeotropic C2ClF5/C3F8 separation continues to rise owing to the significance of ultrapure fluoride gases for the electronic industry. With a boiling point differential of 1.1 K, separating an azeotropic mixture of C2ClF5/C3F8 through sorption‐based processes under ambient conditions is a preferable alternative to the current energy‐intensive cryogenic separation method. Here, we report the highly efficient C2ClF5/C3F8 adsorption separation in quaternary alkylammonium ions modified MFI type zeolites with interlaced channels which provide a cooperative effect conferred by specific interaction sites and optimal pore size. A path‐regulatory mechanism was proposed, whereby TMA+ cations located at intersections resembling “train turnouts” force gas molecules to switch traffic channels to confined sinusoidal channels affecting the C2ClF5/C3F8 recognition sites and diffusion behavior. Molecular simulations were conducted to reveal the separation mechanism, and the breakthrough experiment validated the effectiveness of this adsorbent at 298 K outperforming conventional adsorbents for the challenging azeotropic separation.

Heat transfer in a channel with internal intensifiers during azeotropic mixture circulation

Today, the use of mixtures as refrigerants and coolants in various energy systems is widespread. The volatility coefficients for the components of a non-azeotropic binary mixture vary significantly. In an azeotropic binary mixture, the volatility coefficients for both components are the same. This paper presents the experimental results on the intensity of heat transfer to an azeotropic alcohol-water mixture during forced circulation in a round heated channel. Experiments are carried out both in a smooth channel and in a channel with a spiral intensifier. The inner diameter of the channel is D = 7.6 mm, the length is 2 m. The intensifier surface has a hydrophobic coating. The experiments are carried out at low (0.03 – 0.04 MPa) pressure in the system, in the range of mass velocities 36 < M < 376 kg/m2s and Reynolds numbers of 360 – 3700. The heat flux density in these experiments is 8000 W/m2. In the single-phase flow of an azeotropic alcohol-water mixture in the channel with a spiral intensifier, the heat transfer coefficient is up to 5–6 times higher than that in a smooth channel. In this case, the higher the mass flow velocity, the greater the excess of the heat transfer coefficient. In the two-phase flow of the mixture in the channel with a spiral intensifier, the heat transfer coefficient is up to 2–3 times higher than that in a smooth channel.

ПЕРВАПОРАЦИОННОЕ РАЗДЕЛЕНИЕ ВОДНО-СПИРТОВЫХ СМЕСЕЙ НА ПОЛИУРЕТАНОВЫХ МЕМБРАНАХ НА ОСНОВЕ АМИНОЭФИРОВ БОРНОЙ И ОРТОФОСФОРНОЙ КИСЛОТ: ЭКСПЕРИМЕНТ И МОДЕЛИРОВАНИЕ

In this work, the separation characteristics of a selective layer of pervaporation membranes made of polyurethanes based on amino esters of orthophosphoric acid (AEPA) and amino esters of boric acid (AEBA) were studied during the dehydration of azeotropic water-alcohol mixtures. Polymer compositions of polyurethanes based on various compositions of AEPA and AEBA were studied in order to identify the best options. For polyurethane based on AEPA, moles of orthophosphoric acid influenced the composition of the polymer composition; for polyurethane based on AEBA, the effect of modifiers (bulk adducts of diphenylolpropane diglycidyl ether and copper chloride) was measured. A technology has been developed for the production of polymer pervaporation membranes with a selective layer of restrictive polyurethanes. Experimental studies on the pervaporation dehydration of isopropanol/water and ethanol/water mixtures in various alcohol concentrations (50-99 wt%) and various feedstock temperatures are presented. Obtains permeate flux and separation factor results for membrane confinement when separating confinement mixtures. It has been determined that an increase in the moles of orthophosphoric acid in the composition of the polymer composition AEPA and the use of modifiers in the synthesis of the polymer composition AEBA improve the separation characteristics of pervaporation membranes. It was revealed that when the process temperature increases by 20 °C, the permeate flow through the membrane increases, for example, for polyurethane based on AEPA-5-PEG, the value increases by 2 times, with a practically unchanged separation coefficient. Based on the pervaporation separation index data, promising polymer compositions were selected. The results of the data obtained are compared with the studies of other authors; the membranes we use have high values of permeate flux and separation coefficient, which are consistent with the values for any membranes from the open literature. Based on the concept of dissolution-diffusion, the process of pervaporation of isopropanol/water and ethanol/water mixtures through polymer membranes was simulated. The given calculation system is cyclic, and the results of calculating the permeate flow and separation coefficient using this model are in good agreement with the experimental data obtained.

Comparative Analysis: Trays versus Packed Columns in Pressure-Swing Distillation for the Separation of Tetrahydrofuran, Water and Ethanol Azeotropic Mixture

Comparative Analysis: Trays versus Packed Columns in Pressure-Swing Distillation for the Separation of Tetrahydrofuran, Water and Ethanol Azeotropic Mixture

ВЛИЯНИЕ ЭКСТРАКТИВНЫХ АГЕНТОВ НА ПЛОТНОСТЬ И ВЯЗКОСТЬ ЖИДКОЙ ФАЗЫ ПРИ РЕКТИФИКАЦИИ СМЕСИ ИЗОПРОПАНОЛ-ВОДА

In this work, the dynamic viscosity and density of three-component mixtures consisting of a binary azeotropic mixture of isopropanol-water and an extractive agent were investigated. Three points of the profile of concentrations in the liquid phase in the extractive separation column of isopropanol-water mixture in recalculation on the pseudobinary mixture were selected for research: the first point - 95/5 % wt. %; the second point - 5/95 % wt. %; the third azeotrope point - 88/12 % wt. %. For the selected points model mixtures were prepared to which 13 % wt. % of the extractive agent was added. The extractants used in this work were monoethylene glycol (EG), which is used in industrial processes, 1-ethyl-3-methyl imidazolium chloride ([Emim][Cl]) and triethylene glycol-based boronic acid amino ester (AEBA). Experimental data on concentration and temperature dependences of dynamic viscosity and density were obtained for these mixtures. The results showed that the viscosity of the system increases in the series of EG, AEBA, [Emim][Cl]. To carry out calculations of the investigated thermophysical properties in the work we used known expressions: for density - through the sum of volumes; for viscosity - the Kendall-Manroe model. For a three-component mixture with EG, the results of calculations of properties performed in the AspenPlus V12 package are presented. The studies carried out in this work have shown that the density modeling of the studied mixtures is quite accurate, the discrepancy between experimental and calculated data does not exceed 1%. Modeling of the dynamic viscosity coefficient of the system by the Kendall-Manroe model for isopropanol-water mixture in the presence of the studied extractive agents can lead to significant errors, the maximum value reaches 52%. The calculated values of the dynamic viscosity coefficient are underestimated, which in turn can lead to errors in modeling (on the basis of these properties) of heat and mass transfer processes occurring in the apparatus.

Polar organic cages for efficiently separating azeotropic mixtures.

Dihydroxy-based polar organic cages (DIHO-cages) are reported to selectively separate toluene with 99.5% purity from an equimolar toluene/pyridine mixture, resulting in subsequent superior purification of pyridine. The efficient separation and purification, enhanced by strong and multiple host/guest C-H⋯O interactions between the cage and toluene, showcases DIHO-cages as a suitable candidate for the remarkable separation of such azeotropes on an industrial scale.

Flammability limits of theoretical azeotropic mixtures consist of R1243zf, RE170 and flame retardants (R13I1, R134a, or R227ea)

3,3,3-trifluoropropene (R1243zf) is a low climate change effect refrigerant that has great development potential. However, R1243zf is flammable and its safety classification is A2 as per ISO standard 817. To reduce its risk of flammability, the method of using mixed refrigerants in practical applications is an effective approach. In this paper, an experimental system was established according to the ASHRAE standard 34 to measure the flammability limits of four azeotropic mixtures containing R1243zf. First, the critical suppression ratio (CSR) of R13I1/R1243zf was tested out that is 0.353. Further measurements were made to determine the lower flammability limits (LFLs) and critical suppression concentrations (CSCs) of three ternary mixtures consisted of R1243zf, thermodynamic performance enhancer (RE170) and flame retardants (R13I1, R134a or R227ea), at various compositions. The experimental non-flammable proportions of R1243zf/RE170/R13I1, R1243zf/RE170/R134a, and R1243zf/RE170/R227ea were 31.81 %, 5.49 %, and 22.42 %, respectively. Besides, a comparison was made between the experimental data and the predicted results by means of three different theoretical methods (the equal effect method, thermal balance method, and group contribution method). The fresh experimental results will provide practical guidance for the applications that utilize the R1243zf blends in low flammability risk.

A new hybrid reactive-extractive distillation configuration for ternary azeotropic separation with intensification opportunity

The concept of reactive-extractive distillation (RED) is not new and has gained significant attention as an innovative approach for separating azeotropic mixtures. Presently, there have been at least 18 studies on RED, exploring variations such as triple column RED (TCRED), double column RED (DCRED), and their intensified configurations like dividing-wall DCRED (DW-DCRED) or side-stream DCRED (SS-DCRED). However, a major drawback observed in all existing intensified RED studies is the inability to achieve energy savings, which contrasts with the outcomes of most conventional distillation system studies. To address this, we propose a new alternative configuration called extractive-reactive distillation (ED-RD) by rearranging the column sequence in DCRED. The ED-RD configuration achieves a substantial 7 % reduction in the total annual cost (TAC) compared to DCRED, primarily due to a reduced number of stages and the utilization of relatively cheaper low-pressure steam in the extractive distillation column (EDC). Furthermore, the intensified thermally coupled ED-RD (TC-ED-RD) configuration demonstrates superior performance, achieving an impressive 21 % reduction in TAC compared to the proposed ED-RD configuration, mainly driven by a significant decrease in total reboiler energy consumption. Overall, the TC-ED-RD configuration excels with a remarkable 28 % reduction in total reboiler energy consumption and a 27 % decrease in TAC compared to DCRED. This reflects the potential of incorporating process intensification techniques to the proposed configuration to achieve substantial energy and cost savings, in contrast to the limitations observed in the conventional RED systems.

Exploration of different water content on the performance of reactive-extractive distillation for separating the ternary azeotropic mixture

Reactive-extractive distillation (RED) has gained significant attention as an innovative approach for separating azeotropic mixtures. Although many studies have reported its positive benefits, no studies have explored the impact of varying water content on RED performance, a domain that has seen exhaustive exploration in conventional extractive distillation. In this study, we investigate how water content affects RED performance using the separation of ethyl acetate (EA), 1,4-dioxane (DIO), and water as a case study. We design diverse configurations tailored for feed compositions ranging from 0.35 to 0.8 water mole fractions using a double column reactive-extractive distillation (DCRED). These configurations are globally optimized and the performance is compared against the triple-column conventional extractive distillation (TCCED) based on economic, environmental, and thermodynamic efficiency considerations. The TAC and CO2 emission of TCCED schemes exhibit the linear decreases, reaching its lowest point at 50 mol.% water content. Beyond this threshold, both indicators increase significantly, even surpassing the reference case (20 mol.% water) at 80 mol.% water content. The thermodynamic efficiency of TCCED behaves inversely due to the intrinsic exergy destructions. Interestingly, the DCRED schemes show almost linear decreases in the trends of both economic and environmental indicators, and the notable deviations are then observed at 65 mol.% water content, accompanied with a slight decrease up to 80 mol.%. The thermodynamic efficiency of DCRED scheme gives an inverse trend, identical to those observed in TCRED. Overall, TCCED performs best at 50 mol.%, in which DCRED excels at 80 mol.% water content. This comprehensive study bridges a significant knowledge gap, offering insights into optimizing RED processes for varying water content, an unexplored topic despite widespread application of RED.

STREAMS HEAT INTEGRATION IN THE SEPARATION PROCESS OF THE FURFURAL-WATER HETEROAZEOTROPIC MIXTURE BY USING TWO STRETCHING COLUMNS

The process of azeotropic mixtures rectification takes place in many branches of the chemical industry. This process requires significant energy consumption both for heating and evaporation of process streams, and for vapor condensation and cooling of products. The consumption of external energy essentially depends on the methods of separation processes organization. Pinch analysis is one of the modern methods for designing chemical-technological systems with the aim of optimal use of external energy sources by maximizing the heat integration of system process streams, taking into account the limitations of a particular manufacture, the requirements of environmental safety and environmental protection. In this paper the heat integration of the heteroazeotropic furfural-water mixture separation process by two columns was considered. In each column the azeoptrope is a light-boiling one, and one of the mixture components is a low-boiling one. The heat and material balances of the rectifying plant were calculated and the table of its streams was formed, that is, the data of the chemical-technological system were extracted. From the total set of heat streams, the subset was selected for heat integration. Composite curves for the selected heat streams subject to heat integration were built and their relative position on the temperature-enthalpy diagram was analyzed. This analysis showed that for a certain value of the minimum temperature difference in the heat exchange equipment Тmin, there is a threshold problem. The problem was formulated for a value of Тmin slightly less than the threshold value that led to a slight increase in the consumption of external energy carriers. Options for further improvement of the network of heat exchangers were considered, and the occurrence of a cycle through the pseudopinch was identified. It gave possibility to remove a heat exchanger with a low load and redistribute this load among to other ones. Restoring Тmin in this case is impossible, because a strict restriction on the absence of hot utilities is accepted. Significant savings in external utilities load have been demonstrated. For retrofit modern Alfa Laval heat exchange equipment was selected for all the necessary positions.

Numerical Analysis of Using R449A Refrigerant Alternative to R404A in Cooling Systems: 3E-Analysis (Energetic, Exergetic, and Environmental)

The direct impact of refrigerants on the environment is largely dependent on the magnitude of global warming potential (GWP) values. In this regard, fluids with low GWP are very important for the environment. Commercial refrigeration accounts for about a third of the world's HFC use. R404A is a derived HFC near an azeotropic mixture. The refrigerant of R404A is widely used in commercial refrigeration systems, especially in supermarkets. As a result of the use of R404A in cooling systems with high GWP, both indirect and direct negative effects on the environment increase. In this study, the energy, exergy, and Life Cycle Climate Performance (LCCP) of R449A, which has a lower GWP value, which can be an alternative to R404A refrigerant, were investigated theoretically. Different evaporator temperatures (-30oC to -5oC) and a condenser temperature (30oC) were used in the analysis. According to the results obtained, while the mass flow rate of R449A is approximately 13% lower than that of R404A, its exergy efficiency is 5% higher. R449A was found to be approximately 5% lower than R404A in LCCP analysis.

Extractive Distillation of an Ethyl Acetate–Ethanol–Water Azeotropic Mixture in the Presence of Boric Acid Amino Ester

Extractive Distillation of an Ethyl Acetate–Ethanol–Water Azeotropic Mixture in the Presence of Boric Acid Amino Ester

Extractive distillation of tetrahydrofuran–ethanol azeotropic mixture with ionic liquid as extractant

AbstractBACKGROUNDThe tetrahydrofuran–ethanol azeotrope cannot be separated by common distillation, but can be effectively separated by extractive distillation. Extractive distillation can be simulated by Aspen Plus software. The minimum annual cost of the process can be calculated to assess ionic liquid separation ability. The interaction energy can be used to explain the separation mechanism.RESULTSThe non‐random two‐liquid model parameters of ternary mixtures for tetrahydrofuran and ethanol containing tributylmethylammonium acetate or 1‐octyl‐3‐methylimidazolium acetate were obtained. Thermodynamic property databases of tributylmethylammonium acetate, 1‐hexyl‐2,3‐dimethylimidazolium acetate and 1‐octyl‐3‐methylimidazolium acetate were established. Based on the thermodynamic model parameters and properties, the sequential iterative method was used to determine the best operating conditions and minimum total annual cost the of extractive distillation process. The interaction energy was calculated to explain the separation mechanism.CONCLUSIONSThe separation performance of tributylmethylammonium acetate is better than that of 1‐hexyl‐2,3‐dimethylimidazolium acetate or 1‐octyl‐3‐methylimidazolium acetate. The selection and design of the ionic liquids can be conducted in terms of the interaction energy. © 2023 Society of Chemical Industry (SCI).

Thermodynamic modeling of double azeotropy systems

Azeotropes are boiling liquid mixtures which have identical vapor and liquid compositions. Known as polyazeotropy, azeotropic mixtures may exhibit more than one azeotrope at a given temperature or pressure. While the thermodynamic condition for azeotrope formation in vapor-liquid equilibrium is well established, thermodynamic modeling of mixtures exhibiting polyazeotropy remains challenging and prior modeling studies often failed to satisfactorily correlate the polyazeotropic behavior of associating systems. Based on the association NRTL model, this study presents successful correlation and analysis of double azeotrope formation for three diverse systems: strongly associating methanol + diethylamine binary, weakly associating ammonia + pentafluoroethane binary, and non-associating benzene + hexafluorobenzene binary systems. These systems exhibit polyazeotropic behavior as the temperature increases and approaches their Bancroft points.

The microstructure characteristics of the 1-ethyl-3-methylimidazolium thiocyanate–methyl acetate−methanol systems

The ionic liquid (IL) has demonstrated promising potential as an entrainer in efficiently separating the methyl acetate−methanol azeotropic mixture using extractive distillation. However, comprehending the fundamental mechanism responsible for azeotropism vanished by an IL entrainer is crucial for its large-scale applications. This study aims to explore the microstructure characteristics of the 1-ethyl-3-metfighylimidazolium thiocyanate ([EMIM][SCN])–methyl acetate−methanol systems using FTIR and DFT calculations. The focus is on understanding the fundamental reasons behind azeotrope breaking facilitated by the IL entrainer. Initially, the v(aromatic C–H) region of the imidazolium cation was analyzed to evaluate the interaction strength between the entrainer and methyl acetate/methanol. The results indicated stronger interactions between [EMIM][SCN] and methanol. As a consequence of this variation in interaction strength, the relative vapor pressure of the two constituents was impacted, ultimately resulting in the disturbance of the azeotrope. Subsequently, the v(C=O) region of methyl acetate was utilized to detect interaction complexes in various systems, assisted by excess spectra. Various interaction species, including methyl acetate–methanol/2methanol and methyl acetate–ion cluster/ion pair/cation were found. Notably, when the mole fraction of IL exceeded 0.087, the IL effectively disrupted the interaction complex between methyl acetate and methanol, thereby eliminating azeotropy in the methyl acetate–methanol system.